1. Field of the Invention
The invention relates to a method for regenerating an exhaust gas aftertreatment device in the exhaust system of an internal combustion engine, and more particularly to a particulate filter in the exhaust system of a diesel engine. According to-the method the temperature of the exhaust gases, the hydrocarbon concentration of the exhaust gases, or both are increased by post injection of fuel into the engine. In addition, the invention relates to an internal combustion engine, in particular a diesel engine, designed to carry out the aforesaid method.
2. Background of the Invention
To reduce the harmful emissions from internal combustion engines, exhaust gas aftertreatment devices are placed in the exhaust gas path to process legislated exhaust gas constituents. One such device is a particulate filter which removes unburned particulates of soot from the exhaust gas. Particulate filters are regenerated periodically by burning the accumulated filter residues. The carbon contained in the filter residues ignite only at relatively high temperatures of approximately 550xc2x0 C., if there is no catalytic support. Such temperatures are only reached under conditions with at high engine speeds and torques. To ensure the operation of the particulate filter, measures are taken to increase exhaust gas temperature sufficiently during a regeneration under all operating conditions of the engine.
Various procedures have been proposed for increasing exhaust gas temperature. For example, by switching on electrical loads, it is possible to increase the engine loading. Additional fuel can be injected into the combustion space or into the exhaust gas ducts. As a result, unburned hydrocarbons are fed to a catalytic converter arranged upstream of the particulate filter. These hydrocarbons are oxidized, thereby creating an exotherm in the catalytic converter and raising the exhaust temperature.
Post injection of fuel has proven particularly effective in raising exhaust temperature. Post injection is injection of an additional quantity of fuel into one or all cylinders after the main injection, i.e., during the expansion stroke. Depending on the quantity and the start of the post injection event, a portion of the injected fuel burns in the cylinder and contributes both to engine torque and to an increase in the exhaust gas temperature at the outlet of the engine. The remaining part of the fuel vaporizes and leaves the engine in the form of unburned hydrocarbons. If an oxidation catalytic converter is arranged upstream of the particulate filter and if said oxidation catalyst converter has a sufficiently high temperature, the unburned hydrocarbons oxidize in an exothermic reaction cause a temperature increase in the exhaust gases. However, if the temperature of the oxidation catalytic converter is too low, the hydrocarbons do not oxidize and, thus, leave the exhaust gas system unburned.
While post injection is being carried out, the injected quantity of fuel, and in particular the time of the injection, are to be matched very precisely in to ensure that:
the maximum temperature limits of the turbine and of the exhaust system are not exceeded;
the enthalpy release rate in the catalytic converter does not exceed acceptable limits;
emission of unburned hydrocarbons does not exceed acceptable limits;
additional torque generated by the post injection does not cause the total generated torque to exceed desired torque.
While compliance with all these conditions is already demanding under normal conditions, an aggravating factor is that, in practice, ambient conditions vary considerably. These affect the conditions (temperature, pressure) which prevail in the cylinder at the time of post injection and have a decisive influence on the proportion of post-injected fuel which is burned. An excessively high proportion of unburned hydrocarbons can lead to cooling of the oxidation catalytic converter to below the threshold temperature for oxidation. This leads to an additional drop in the exhaust gas temperature so that the desired regeneration of the particulate filter does not occur. In addition, an excessively high concentration of hydrocarbons downstream of the catalytic converter leads to an unpleasant unacceptable smell. The torque may also be less in such a situation than is expected on the basis of the position of the accelerator pedal.
Against this background, the present invention provides a method for regenerating an exhaust gas aftertreatment device which enables the effects of a post injection to be stabilized. In particular, the present invention is a method for regenerating an exhaust gas aftertreatment device disposed in the exhaust system of an internal combustion engine, the engine having a throttle valve disposed in the engine intake and a fuel injector disposed in a combustion cylinder of the engine capable of multiple injections during a combustion cycle. Fuel is injected into the combustion cylinder at least twice in a single engine cycle, including a main injection followed by a post injection. The throttle valve is regulated to provide a desired value of an exhaust gas parameter.
In one embodiment, the exhaust gas parameter is exhaust gas temperature. Alternatively, the exhaust gas parameter is exhaust gas hydrocarbon concentration.
The proposed method for regenerating an exhaust gas aftertreatment device in the exhaust system of an internal combustion engine may be used in particular for regenerating a particulate filter in the exhaust system of a diesel engine. In the method, the temperature of the exhaust gases and/or the concentration of the unburned hydrocarbons in the exhaust gas are increased by a post injection of fuel in the working cycle of the internal combustion engine. The method is defined by virtue of the fact that the values predetermined by the engine controller for the intake pressure (MAP) and/or for the air mass flow rate (MAF) are changed as a function of a variable whose value is correlated with the ambient temperature of the engine or of the internal combustion engine, in such a way that stable values of the exhaust gas temperature and/or of the concentration of hydrocarbons in the exhaust gas are achieved by the post injection.
The method according to the invention thus achieves stable effects of the post injection with respect to the exhaust gas temperature and the concentrations of hydrocarbons by virtue of the fact that the ambient temperature of the engine or of the motor vehicle is taken into account in the setting of the intake pressure and/or of the air mass flow rate. They are taken into account here in the form of a correction of the values predetermined by the engine controller on the basis of other, conventional criteria. It has become apparent that such a measure can enable a reliable regeneration of the exhaust gas aftertreatment device, for example of the particulate filter in the exhaust gas system of a diesel engine, to take place even under greatly varying ambient conditions. Here, the strategies used for the stabilization are relatively simple, i.e., regulating the intake pressure or air mass flow rate. An advantage of the present method is that complex measures, for example preheating of the supplied air mass flows to ensure uniform temperature in the combustion space is not necessary.
The predetermined values for the intake pressure and/or the air mass flow rate can be changed in such a way that at the time of the post injection, the temperature in the combustion space of the internal combustion engine does not drop below a predetermined threshold. It has become apparent that the temperature in the combustion space at the time of the post injection has a decisive influence on how complete combustion of the post-injected quantity proceed or whether fuel leaves the engine without being burned. By regulating the throttle valve, temperature in the combustion chamber is regulated in such a way that it does not drop below a specific threshold so that the desired exhaust gas temperature and/or the concentration of the hydrocarbons can be achieved.
A variety of indirect variables which are correlated to the ambient temperature are available for determining the ambient temperature. However, the ambient temperature is preferably measured directly by a suitable sensor.
According to one specific refinement of the method according to the invention, the lower the ambient temperature of the engine or of the motor vehicle the greater the reduction in the predetermined values for the intake pressure and/or for the air mass flow rate. The reduction in the values for the intake pressure or the air mass flow rate causes the combustion of the quantity of fuel from the main injection to take place more slowly so that relatively high temperatures still prevail in the combustion chamber at the time of the post injection. This measure is therefore advantageously set at a higher level the lower the ambient temperature.
According to one development of the method, the predetermined values for the intake pressure and/or for the air mass flow rate are additionally corrected if the greater operating temperature of the internal combustion engine has not yet been reached. The operating temperature of the internal combustion engine can be based on coolant temperature or cylinder head temperature, both of which are continuously measured and monitored in any case. Operating temperature of the engine constitutes a further decisive influencing parameter indicating temperature in the cylinder at the time of post injection of fuel.
According to another development of the invention, the predetermined values for the intake pressure and/or the air mass flow rate are reduced when the engine load is low in such a way that stable values of the exhaust gas temperature and/or of the concentration of hydrocarbons are obtained in the exhaust gas as a result of the post injection. That is, even a low engine load can lead to a reduction in the intake pressure/air mass flow rate independently of, or in addition to, the temperature of the surroundings. It is therefore advantageous if a relatively high temperature is generated in the combustion chamber by reducing the intake pressure or the air mass flow rate at the time of the post injection.
The predetermined values for intake pressure and/or air mass flow rate can be determined in the conventional way by an engine controller. In particular, they can be determined as a function of engine speed and engine load or desired engine torque.
To ensure good driving characteristics during the regeneration of the exhaust gas aftertreatment device, the throttling, brought on by the change in the predetermined values for the intake pressure and/or the air mass flow rate, can be reduced or entirely eliminated if a high degree of acceleration and/or a high engine load is present or desired.
The invention also relates to an internal combustion engine with an exhaust gas aftertreatment device arranged in the exhaust system, and with a control device for regenerating the exhaust gas aftertreatment device by increasing the temperature and/or the concentration of hydrocarbons in the exhaust gases. The internal combustion engine can be, in particular, a diesel engine, and the exhaust gas aftertreatment device can be, in particular, a particulate filter. The control device is configured in such a way and connected to sensors and actuators in such that the latter can carry out the method of the present invention.
The control device contains, in particular, a unit for registering a variable which is correlated to the ambient temperature, and that the control unit can influence the intake pressure which is set and/or the air mass flow rate. This influence is exerted as a function of the aforesaid correlating variable in such a way that predetermined values for the intake pressure and/or the air mass flow rate are changed in such a way that stable values of the exhaust gas temperature and/or of the concentration of hydrocarbons in the exhaust gas are achieved by the post injection.
The control device is preferably connected to a temperature sensor measuring temperature of the engine and/or of the motor vehicle and/or temperature of the charge air and/or ambient temperature. This temperature is taken into account in the change in the intake pressure and/or in the air mass flow rate.